This warning is to notify you that grades will not be retroactively changed and no data will be lost. If the value associated with a grade was changed and is used in calculations, the calculation result may be affected. So if you change an A to have a cutoff value of 96 instead of 98, this will not change grades that exist on student records currently. But if you try to then recalculate a GPA using this new translation, the results may be different than before.